The invention relates to a safety coupling comprising two coupling disks pressed against each other by a spring device, in particular for employment in screwing tools used for tightening a screw at a predetermined torque.
In DE 296 18 871 U a safety coupling in described which comprises two coupling disks accommodated in a housing. The coupling disks are provided with engaging profiles and are pressed against each other by a spring device. When the screwing resistance of a screw to be tightened is larger that the adjusted trip moment of the safety coupling, the profiles disengage so that the first coupling disk rotates. In this way the torque to be applied can be limited. However, the contiguous profiles of the two coupling disks cause friction which affect the trip moment in an unforeseeable way. On the one hand it must be taken into consideration that the static friction is larger than the sliding friction caused by displacement of the profiles, and further that the sliding friction may vary depending on the material and lubricant properties.
Further, safety couplings are known which comprise rolling bodies in the form of balls or rollers between the two coupling disks. However, balls are suitable only for low trip moments because of their point contact with the adjacent coupling disk since they do not withstand larger pressure forces or cause bulging-in of the coupling disk concerned. The rollers pose the problem that the rollers are aligned radially with the coupling disks and that the support along the inner radius does not conform to that on the outer radius. If the troughs of the coupling track have parallel boundary edges corresponding to the form of the cylindrical rollers, the boundary edges of the troughs do not extend radially to the respective coupling disk so that the roller leaves one end of the trough earlier than the other end. Consequently, the carrying capacity of the rollers can be utilized only up to a certain extent, and when a high load is applied the coupling track may be damaged and thus the torque residence time and accuracy adversely affected.